Potential memory leaks fixed.

src/main/java/io/github/retrooper/packetevents/PacketEvents.java

@@ -33,6 +33,7 @@ import io.github.retrooper.packetevents.processor.PacketProcessorInternal;
 import io.github.retrooper.packetevents.settings.PacketEventsSettings;
 import io.github.retrooper.packetevents.updatechecker.UpdateChecker;
 import io.github.retrooper.packetevents.utils.entityfinder.EntityFinderUtils;
+import io.github.retrooper.packetevents.utils.guava.GuavaUtils;
 import io.github.retrooper.packetevents.utils.netty.bytebuf.ByteBufUtil;
 import io.github.retrooper.packetevents.utils.netty.bytebuf.ByteBufUtil_7;
 import io.github.retrooper.packetevents.utils.netty.bytebuf.ByteBufUtil_8;
@@ -124,6 +125,8 @@ public final class PacketEvents implements Listener, EventManager {
 
                 EntityFinderUtils.load();
 
+                getServerUtils().entityCache = GuavaUtils.makeMap();
+
                 if (version.isNewerThanOrEquals(ServerVersion.v_1_9)) {
                     for (WrappedPacketOutEntityEquipment.EquipmentSlot slot : WrappedPacketOutEntityEquipment.EquipmentSlot.values()) {
                         slot.id = (byte) slot.ordinal();

src/main/java/io/github/retrooper/packetevents/utils/guava/GuavaUtils.java

@@ -0,0 +1,37 @@
+/*
+ *
+ *  * This file is part of packetevents - https://github.com/retrooper/packetevents
+ *  * Copyright (C) 2016-2021 retrooper and contributors
+ *  *
+ *  * This program is free software: you can redistribute it and/or modify
+ *  * it under the terms of the GNU General Public License as published by
+ *  * the Free Software Foundation, either version 3 of the License, or
+ *  * (at your option) any later version.
+ *  *
+ *  * This program is distributed in the hope that it will be useful,
+ *  * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  * GNU General Public License for more details.
+ *  *
+ *  * You should have received a copy of the GNU General Public License
+ *  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+package io.github.retrooper.packetevents.utils.guava;
+
+import io.github.retrooper.packetevents.PacketEvents;
+import io.github.retrooper.packetevents.utils.server.ServerVersion;
+
+import java.util.concurrent.ConcurrentMap;
+
+public class GuavaUtils {
+    public static <T, K>ConcurrentMap<T, K> makeMap() {
+        if (PacketEvents.get().getServerUtils().getVersion().isNewerThan(ServerVersion.v_1_7_10)) {
+            return GuavaUtils_8.makeMap();
+        }
+        else {
+            return GuavaUtils_7.makeMap();
+        }
+    }
+}

src/main/java/io/github/retrooper/packetevents/utils/guava/GuavaUtils_7.java

@@ -0,0 +1,31 @@
+/*
+ *
+ *  * This file is part of packetevents - https://github.com/retrooper/packetevents
+ *  * Copyright (C) 2016-2021 retrooper and contributors
+ *  *
+ *  * This program is free software: you can redistribute it and/or modify
+ *  * it under the terms of the GNU General Public License as published by
+ *  * the Free Software Foundation, either version 3 of the License, or
+ *  * (at your option) any later version.
+ *  *
+ *  * This program is distributed in the hope that it will be useful,
+ *  * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  * GNU General Public License for more details.
+ *  *
+ *  * You should have received a copy of the GNU General Public License
+ *  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+package io.github.retrooper.packetevents.utils.guava;
+
+import net.minecraft.util.com.google.common.collect.MapMaker;
+
+import java.util.concurrent.ConcurrentMap;
+
+class GuavaUtils_7 {
+    static <T, K> ConcurrentMap<T, K> makeMap() {
+        return new MapMaker().weakValues().makeMap();
+    }
+}

src/main/java/io/github/retrooper/packetevents/utils/guava/GuavaUtils_8.java

@@ -0,0 +1,31 @@
+/*
+ *
+ *  * This file is part of packetevents - https://github.com/retrooper/packetevents
+ *  * Copyright (C) 2016-2021 retrooper and contributors
+ *  *
+ *  * This program is free software: you can redistribute it and/or modify
+ *  * it under the terms of the GNU General Public License as published by
+ *  * the Free Software Foundation, either version 3 of the License, or
+ *  * (at your option) any later version.
+ *  *
+ *  * This program is distributed in the hope that it will be useful,
+ *  * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  * GNU General Public License for more details.
+ *  *
+ *  * You should have received a copy of the GNU General Public License
+ *  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+package io.github.retrooper.packetevents.utils.guava;
+
+import com.google.common.collect.MapMaker;
+
+import java.util.concurrent.ConcurrentMap;
+
+class GuavaUtils_8 {
+    static <T, K> ConcurrentMap<T, K> makeMap() {
+        return new MapMaker().weakValues().makeMap();
+    }
+}

src/main/java/io/github/retrooper/packetevents/utils/server/ServerUtils.java

@@ -33,7 +33,8 @@ public final class ServerUtils {
     private boolean geyserClassChecked;
     private final NPCManager npcManager = new NPCManager();
 
-    private final Map<Integer, Entity> entityIDs = new ConcurrentHashMap<>();
+    //Initialized in PacketEvents#load
+    public Map<Integer, Entity> entityCache;
 
     /**
      * Get the server version.
@@ -86,11 +87,11 @@ public final class ServerUtils {
 
     @Nullable
     public Entity getEntityById(int entityID) {
-        return entityIDs.get(entityID);
+        return entityCache.get(entityID);
     }
 
     public void cacheEntityById(int entityID, @NotNull Entity entity) {
-        entityIDs.putIfAbsent(entityID, entity);
+        entityCache.putIfAbsent(entityID, entity);
     }
 
     public boolean isGeyserAvailable() {